The Minimum Admissible Detuning Efficiency of MRI Receive-Only Surface Coils.

BACKGROUND: The minimum admissible detuning efficiency (DE) of a receive coil is an essential parameter for coil designers. A receive coil with inefficient detuning leads to inhomogeneous B1 during excitation. Previously proposed criteria for quantifying the DE rely on indirect measurements and are difficult to implement.

PURPOSE: To present an alternative method to quantify the DE of receive-only surface coils.

STUDY TYPE: Theoretical study supported by simulations and phantom experiments.

PHANTOMS: Uniform spherical (100 mm diameter) and cylindrical (66 mm diameter) phantoms.

FIELD STRENGTH/SEQUENCE: Dual repetition time B1 mapping sequence at 1.5T, and Bloch-Siegert shift B1 mapping sequence at 3.0T.

ASSESSMENT: One non-planar (80 × 43 mm2 ) and two planar (40 and 57 mm diameter) surface coils were built. Theoretical analysis was performed to determine the minimum DE required to avoid B1 distortions. Experimental B1 maps were acquired for the non-planar and planar surface coils at both 1.5T and 3.0T and visually compared with simulated B1 maps to assess the validity of the theoretical analysis.

STATISTICAL TESTS: None.

RESULTS: Based on the theoretical analysis, the proposed minimum admissible DE, defined as DEthr  = 20 Log (Q) + 13 dB, depended only on the quality factor (Q) of the coil and was independent of coil area and field strength. Simulations and phantom experiments showed that when the DE was higher than this minimum threshold level, the B1 field generated by the transmission coil was not modified by the receive coil.

DATA CONCLUSION: The proposed criterion for assessing the DE is simple to measure, and does not depend on the area of the coil or on the magnetic field strength, up to 3T. Experimental and simulated B1 maps confirmed that detuning efficiencies above the theoretically derived minimal admissible DE resulted in a non-distorted B1 field.

EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 1.